Charles E. Corry and Miller L. Bell © 1968, 1999
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The letter of rejection from Nature for the following article is dated August 28, 1968. At the time most earth scientists would not even accept the fact that meteorites regularly impacted the earth. For example, Barringer Crater in Arizona was still thought by many to be of volcanic origin, as well as the craters on the moon. Bob Dietz had just published his work on shatter cones but I wouldn't say that had been generally accepted. There was not even general agreement on sea floor spreading and plate tectonics outside the radical few at Scripps, Woods Hole, and related institutions.
I went back to the Guatemala Basin a couple of times doing terrestrial heat flow studies. The flux was consistently low, and sometimes negative even though the basin lies just east of the East Pacific Rise. Sea floor spreading theory dictates that the terrestrial heat flux be quite high near the ridge crest. As I haven't seen a better explanation for our heat flow results, and meteorite impacts are now generally accepted, there may still be some merit to our hypothesis. Subsequent expeditions have generally confirmed the extremely low heat flow that we found.
I have made no attempt here to update the references or further support our argument although a couple of comments in parentheses have been added.
Woods Hole Oceanographic Institution Contribution 2134.
It has been known for some time that the Guatemala Basin is a large area of low terrestrial heat flow (Bullard and others, 1956, Von Herzen, 1959, Von Herzen and Uyeda, 1963, Vacquier and others, 1967). The cause has been ascribed to various special conditions of a low radioactive content in the crust and mantle beneath the basin, or the presence of a down-turning convection cell beneath it. As we pointed out in a review of heat flow values from the eastern Pacific (Vacquier and others, 1967) these theories seem inadequate to explain the low terrestrial heat flow found within this basin.
Rather than invoke an anomalous crust and mantle condition, it occurs to us from the roughly circular shape of the basin, and the distribution of the low heat-flow values, Figure 1, that the basin may be a meteorite crater. The low heat flow is then explained as being the result of the blanketing effect of rubble and sediment that falls back, or is washed back into the crater by the influx of the oceans after the explosion. A blanketing layer from 0.5 to 5 km thick would significantly reduce the observed heat flow for from 1 to 10 my (Von Herzen and Uyeda, 1963). The inverse of this relationship between cratering and heat flow was noted by Dietz (1963) in the Ries Basin.
A seismic refraction station near the center of the basin shows a 4.32 km/s layer that is 1.5 km thick (Station 8, Figure 1 and Figure 2) overlaying the crust (Raitt, R., Seismic refraction studies on Chubasco expedition, unpublished [at the time]). We propose that this represents our blanketing layer.
From the work done by Urey (1957) on the frequency of cometary impacts, and Brown (1960) and Baldwin (1963) on the frequency of meteorite impacts, it appears reasonable that within the age of the ocean basins, 10 8 years, that at least one to three extraterrestrial bodies with sufficient energy to cause such a crater have collided with the earth.
These statistics would make it improbable that such an impact has occurred with the past few m.y. However, from the age range of known tektites, and assuming that tektites are splash from large energy meteorite impacts (Urey, 1957, Lin, 1966, and now generally accepted), whether of terrestrial or moon origin, we should like to argue that the frequency of large energy impacts may be much greater that previously supposed (Nininger,1963, and now well established).
If tektites do originate on the moon then for every impact on the moon capable of ejecting tektite material there must be, on average, at least 13.6 impacts on the earth based on relative cross-sections. Brown (1960) adds ~30% more terrestrial impacts to the above cross section figures to account for the larger gravity effects of the earth. Glass (1967) and Glass and Heezen (1967) have shown a possible relationship between tektites and geomagnetic reversals. If this relationship were valid throughout the age of the ocean basins, then ~200 impacts (Heirtzler and others, 1968) of energies ≥ 10 30 ergs (Lin, 1966) have occurred in the last 10 8 years.
Due to the tsunami effects of shock radiating through the crust and mantle, a crater the size of the Guatemala Basin would completely alter an area at least twice its diameter, or ~3x10 6 km 2 . Two hundred such events in 10 8 years are then more than adequate to have created the ocean basins on the basis of area cratered and altered by cratering. This impact frequency is also in approximate agreement with that calculated by Cohenour and Sharp (1968). The time-frequency boundaries above are not necessarily limiting for a hypothesis of formation of the ocean basins by impact, as second order effects may well serve to give ocean basins an appearance of youth with a greatly reduced frequency of impacts, as has been postulated by Harrison (1960).
Some relevant statistics on our proposed crater: mean diameter ~1,100 km; energy required ~10 35 ergs (Chapman and Gault, 1967). Assuming an impact velocity of 42 km/sec (Urey, 1957) the diameters of bodies required to produce this amount of energy are: (1) stony meteorite with density of 3.5 gm/cm 3 , 85 km; (2) iron meteorite with a density of 8 gm/cm 3 , 65 km; and for a comet with a mean density of 0.01 gm/cm 3 , 600 km. The original crater depth would be on the order of 150 km (Chabai, 1965) with a residual depth of roughly 25 km. The time for such a crater to reach isostatic equilibrium would be ~40,000 years (Haskell, 1937). A crater ~1,000 km in diameter and ~150 km deep would lower sea level ~320 m. At the residual crater depth of ~25 km, sea level would still be lowered ~55 m. It should be noted that large uncertainties exist in the above figures and the approximations are probably not better than ±25%.
The earth must be as frequently cratered per unit area as the moon. By a relative cross section argument, more than 13 times the number of craters the size of the maria on the moon exist, or existed, on the earth. Whether such events occur with sufficient frequency in recent geologic time to provide tangible evidence today of such cratering is uncertain. From the arguments set forth, and the continuing discovery of meteorite craters on the continents (Short, 1966, Baldwin, 1963, Dietz, 1961, and Prouty, 1952) it seems likely that the importance of the effect of extraterrestrial bodies impacting the earth has been, at least, underestimated (the Alverez's hypothesis concerning the end of the dinosaurs by such a mechanism was more than a decade in the future). Certainly there is as much evidence at present to support our hypothesis for the formation of the Guatemala Basin as other hypotheses advanced to explain the low heat flow found in this basin.
We should like to thank Dr. Russell Raitt for allowing us to use previously unpublished data. The work was partially supported by NSF grants GM-7 and GP-3791 at Scripps Institution of Oceanography, and NSF grant GA-1448 at the Woods Hole Oceanographic Institution.